Since the invention of the transistor in the late 1940s, tremendous advances have been made in the field of microelectronics. Current technology allows for the cost-effective fabrication of integrated circuits (ICs) with over 100 million components--all on a piece of silicon roughly 10 mm on a side. The one billion transistor IC will be commercially available within a few years. The desire for greater functionality and performance at less cost per IC drives several trends.
First, functionality drives IC transistor counts up. Second, the transistors themselves are being reduced in size so as to achieve greater packing density and, very importantly, to improve their performance. As far as performance is concerned, the key parameter for Metal-Oxide-Semiconductor-Field-Effect-Transistors (MOSFETs, the dominant transistor technology of the day) is the channel length. The channel length (L) is the distance that charge carriers must travel to pass through the device, and a reduction in this length simultaneously implies higher current drives, reduced parasitic resistances and capacitances and improved high-frequency performance. A common figure-of-merit is the power-delay product, and this generalized measure of transistor performance improves as the cube of the inverse of the channel length (1/L.sup.3). This explains the tremendous incentive that IC manufacturers have to reduce the channel length as much as manufacturing capabilities will allow.
For digital applications, MOS transistors behave like switches. When `on`, they drive relatively large amounts of current, and when turned `off` they are characterized by a certain amount of leakage current. As channel lengths are reduced, drive currents increase, which is beneficial for circuit performance as stated above. However, leakage currents increase as well. Leaky transistors contribute to quiescent power dissipation (the power dissipated by an IC when idle) and in extreme cases can affect the transfer of binary information during active operation. Device designers therefore have good reason to keep leakage currents low as channel lengths are reduced.
MOS transistor leakage currents are traditionally controlled by introducing controlled amounts of impurities (dopants) into the channel region of the device, and by tailoring the source/drain lateral and vertical doping distributions. Although these approaches are effective in shoring up the potential barrier internal to the MOS transistor and therefore reducing the leakage current, they can also contribute to degraded drive current and increased parasitic capacitance--the very items that channel length reduction is meant to improve. Furthermore, depending on exactly how in the manufacturing process the channel and tailored source/drain dopants are introduced, the manufacturing cost can be affected significantly. Given traditional MOS transistor design and architecture, there are only limited solutions to the trade-off between drive current, leakage current, parasitic capacitance and resistance, and manufacturing complexity/cost.
The present invention offers a new relationship between these competing requirements, and makes possible MOS devices with characteristics that are not achievable with traditional (impurity doped) MOS architectures. The use of metal for the source and drain and a simple, uniformly implanted channel dopant profile provides for improvements to device characteristics in terms of reduced parasitic capacitance, reduced statistical variations in these characteristics (especially as the channel length is decreased) and reduced manufacturing cost and complexity.